Large Eddy Simulation of Turbulent Combustion in PPC and Diesel Engines

Abstract: This thesis deals with large eddy simulations (LES) of turbulent combustion processes in direct injection internal combustion (IC) engines. Modeling of direct injection IC engine combustion involves modeling of turbulent spray/gas two-phase flow, modeling of chemical reactions of large hydrocarbon fuels and coupling of chemistry with turbulent flows. LES is chosen in this thesis for it capability to resolve the turbulent structures and the fuel and air mixing, and to provide high spatial and temporal resolution of the unsteady fluid motions and the reaction processes. Lagrangian particle tracking (LPT) method is used to model the fuel spray. Finite rate chemistry is employed with a computationally efficient integration method, the chemistry coordinate mapping (CCM) method, which allows the use of large chemical reaction mechanisms with low computational time. The thesis consists of three parts. In part I the feasibility and accuracy of the LES/LPT approach are investigated for simulation of fuel spray injection, evaporation, and mixing with the ambient gas in high-pressure constant volume vessels known as the rigs of the Engine Combustion Network (ECN). The fuel and air mixing, the liquid penetration length and the vapor-phase fuel jet penetration predicted by the LES/LPT model are in good agreement with the experimental data and the importance of stochastic turbulence dispersion (STD) and spray-induced turbulence (SIT) is investigated. In part II the fuel/air mixing, ignition, liftoff and stabilization of diesel flames in modern diesel engines are studied. Diesel engines often employ multiple-hole injectors and inject at high pressures to control the rate of combustion. To deepen the knowledge of the combustion processes in modern diesel engines, LES is used to investigate the effect of inter-jet angles on the auto- ignition, lift-off length, and the wall effect on the mixing and soot formation process. For the three inter-jet angles employed, 45°, 90° and 135°, a clear inter-jet angle effect is captured, with the trends found in previous experiments properly predicted. From the simulations, the effect such as a shortened lift-off length with decreased inter-jet angle and the effect of varying inter-jet angles on the equivalence ratio in the near-wall regions are scrutinized. In part III, LES is employed to simulate the fuel and air mixing process, the onset of low temperature and high temperature ignition in light-duty and heavy-duty partially premixed combustion (PPC) engines. The PPC engine relies on the fuel and air charge stratification to control the combustion event. With LES the effects of umbrella angles, multiple injection strategies, mean swirling flow, and turbulence, on the mixing process and combustion process are simulated. The LES results reveal the detailed mixing process at different injection condition. The results indicate the high sensitivity of the performance of PPC engines to the operating condition and engine configurations, e.g., injection timing, fuel split and injection directions. The results provide insight into the complex physical and chemical process involved, which helps improving the understanding of the performance of PPC engines.